[FFmpeg-devel] [PATCH v2 1/3] [GSoC] [AAC] aaccoder: Implement Perceptual Noise Substitution
klaussfreire at gmail.com
Tue Apr 14 17:42:21 CEST 2015
On Tue, Apr 14, 2015 at 6:10 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Tue, Apr 14, 2015 at 12:33:50AM +0100, Rostislav Pehlivanov wrote:
>> This commit implements the perceptual noise substitution AAC extension. This is a proof of concept implementation, and as such, is not enabled by default. This is the second revision of this patch, made after some discussion via non-public email due to a mistake. Any changes made since the first revision have been indicated.
>> In order to extend the encoder to use an additional codebook, the array holding each codebook has been modified with two additional entries - 13 for the NOISE_BT codebook and 12 which has a placeholder function. The cost system was modified to skip the 12th entry using an array to map the input and outputs it has. It also does not accept using the 13th codebook for any band which is not marked as containing noise, thereby restricting its ability to arbitrarily choose it for bands. The use of arrays allows the system to be easily extended to allow for intensity stereo encoding, which uses additional codebooks.
>> The 12th entry in the codebook function array points to a function which stops the execution of the program by calling an assert with an always 'false' argument. After a discussion, it was pointed out in an email discussion with Claudio Freire that having a 'NULL' entry can result in unexpected behaviour and could be used as a security hole. There is no danger of this function being called during encoding due to the codebook maps introduced.
>> Another change from version 1 of the patch is the addition of an argument to the encoder, '-aac_pns' to enable and disable the PNS. This currently defaults to disable the PNS, as it is experimental. The switch will be removed in the future, when the algorithm to select noise bands has been improved. The current algorithm simply compares the energy to the threshold (multiplied by a constant) to determine noise, however the FFPsyBand structure contains other useful figures to determine which bands carry noise more accurately.
>> Finally, the way energy values are converted to scalefactor indices has changed since the first commit, as per the suggestion of Claudio Freire. This may still have some drawbacks, but unlike the first commit it works without having redundant offsets and outputs what the decoder expects to have, in terms of the ranges of the scalefactor indices.
>> Some spectral comparisons: https://0x0.st/T7.png (original), https://0x0.st/Th.png (encoded without PNS), https://0x0.st/A1.png (encoded with PNS, const = 1.2), https://0x0.st/Aj.png (spectral difference). The constant is the value which multiplies the threshold when it gets compared to the energy, larger values means more noise will be substituded by PNS values. Example when const = 2.2: https://0x0.st/Ae.png
> its probably better to upload and link to places that are more permanent
> than 0x0.st as someone in the future might see this discussion or
> commit and could want to see the pictures too
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